The present invention relates to a method of and apparatus for depositing molten solder on a printed circuit board (PCB).
Conventionally, solder for bonding surface mount technology (SMT) devices to a PCB is deposited on the PCB in the form of a paste. The paste is usually deposited via a stencil of an area commensurate with the PCB. Apertures are formed in the stencil at points corresponding to solder pads on the PCB. The paste is initially placed on the stencil surface remote from the PCB and forced through the apertures onto the PCB by a squeegee. The PCB is passed through a furnace in which the solder paste melts, wets the pads and the leads of devices are placed on the PCB. The solder then solidifies and bonds the devices to the pads. In general, acceptable solder joints are produced if the deposited paste exceeds a minimum height of about 0.2 mm. In practice, if the solder pads vary in size (typically from 0.25 mm height upwards), the conventional stencil technique is usually augmented by additional solder deposition processes to ensure that acceptable amounts of solder are deposited on all of the solder pads. However, these additional processes introduce unwanted manufacturing delays.
IBM Technical Disclosure Bulletin Vol. 33 No. 12, describes a tool for depositing molten solder on a printed circuit board. The tool comprises a container for carrying molten solder. The container has a faceplate perforated by cylindrical holes and a piston for regulating the flow of solder through the holes. The holes are arranged to match the pad pattern of a device site on a PCB. In use, the tool is positioned over a receiving device site and the solder is urged onto the site by the piston. The tool is then moved to another receiving device site (pad) on the PCB. Because the solder is molten rather than in paste form, it can be dispensed through smaller apertures and onto smaller pads. Because the tool must be moved from one device site to another, the molten solder can be dragged across the PCB thereby producing unwanted solder bridges between adjacent pads.